In a high-frequency and large-capacity optical fiber communication system, optical modulators including a waveguide-type optical modulation element have been widely used. With regard to the optical modulators, an optical modulation element, in which LiNbO3 (hereinafter, also referred to as “LN”) having an electro-optic effect is used for a substrate, is widely used in a high-frequency and large-capacity optical fiber communication system when considering that it is possible to realize wide-band optical modulation characteristics with a low optical loss.
In the optical modulation element using the LN, for example, a Mach-Zehnder type optical waveguide is formed on the LN substrate, and when a high-frequency signal is applied to an electrode formed on the optical waveguide, a modulated signal light beam (hereinafter, referred to as “modulated light beam”) corresponding to the high-frequency signal is output. In addition, in a case of using the optical modulation element in an optical transmission device, an optical modulation device, which includes a housing in which the optical modulation element is accommodated, an input optical fiber through which a light beam from a light source is input to the optical modulation element, and an output optical fiber that guides the light beam output from the optical modulation element to the outside of the housing, is used.
With regard to a modulation mode in the optical fiber communication system, a transmission format in which polarization multiplexing is introduced becomes a mainstream from the recent trend of an increase in transmission capacity. Examples of the transmission formation include dual polarization-quadrature phase shift keying (DP-QPSK), dual polarization-quadrature amplitude modulation (DP-QAM), and the like in which two linearly polarized light beams in polarization directions orthogonal to each other are subjected to phase shift keying or quadrature amplitude modulation and are transmitted by one optical fiber.
In the optical modulation device that performs the DP-QPSK modulation or the DP-QAM modulation, a linearly polarized light beam output from one light source is input to the optical modulation element. In the optical modulation element, the input linearly polarized light beam is branched into two light beams, and the two light beams are modulated by using two independent high-frequency signals. The two linearly polarized modulation light beams, which are modulated, are combined into one light beam and the resultant light beam is output in a state of being coupled to one optical fiber.
On the other hand, for example, consideration is given to the following wavelength multiplexing system so as to further increase transmission capacity of the optical transmission system. Specifically, the DP-QPSK modulation or the DP-QAM modulation is performed with respect to a plurality of light beams having wavelengths different from each other, and the plurality of modulated light beams having wavelengths different from each other are collected as one light beam by using a wavelength combiner. The resultant light beam is transmitted by one optical fiber. In an optical transmission device in which a plurality of light beams are respectively modulated and are transmitted by one optical fiber, from the viewpoint of miniaturization of the device and the like, there is a demand for an integration-type optical modulation device that includes a plurality of optical modulation elements (or an integration-type optical modulation element in which a plurality of optical modulation elements are formed on one LN substrate) in one housing to respectively modulate the plurality of input light beams and output the plurality of modulated light beams.
In this case, in general, it is necessary to extend a distance between two linearly polarized light beams output from one optical modulation element and two linearly polarized light beams output from another optical modulation element from the necessity for securing a space for optical components such as a polarization beam combiner that combines light beams (linearly polarized light beams) output two by two from each of the plurality of optical modulation elements, and a lens that couples light beams output from the polarization beam combiner to an optical fiber.
As the integration-type optical modulation device, in the related art, there is known the following integration-type optical modulation device including two optical modulation elements. After a distance between two linearly polarized light beams output from one optical modulation element and two linearly polarized light beams output from the other optical modulation element is extended by using two optical path shift prisms (that is, prisms for parallel movement of an optical path), the two linearly polarized light beams output from each of the optical modulation element are combined into one light beam by a polarization beam combining prism and the like, and the resultant light beam is output to the outside of a housing by one optical fiber (Patent Literature No. 1).
In the optical modulation device, distances from the two optical modulation elements to the two optical path shift prisms are made to be different from each other. According to this, damage of an optical component, which occurs when the two optical path shift prisms come into contact with each other and the like, is prevented.
However, in a case of constituting the optical modulation device, it is required to significantly reduce the number of optical components which are inserted into an optical path as possible from the viewpoint of an improvement in optical coupling efficiency between the optical modulation element and an output optical fiber, from the viewpoint of stabilization of a temperature variation or a variation with the passage of time in the optical coupling efficiency, and from the viewpoint of a reduction in a device size or a device cost.
That is, in the integration-type optical modulation device of the related art, there is a room for improvements from the viewpoints of an improvement and stabilization of optical characteristics, miniaturization, low cost, and the like.